Electrochemical capacitors typically include outer substrates upon which electroactive material is disposed. Both outer substrates and electroactive material disposed thereon can be formed from equivalent materials, in which case the capacitor is "symmetrical", or different materials, in which case the capacitor is "asymmetrical." Each outer substrate and its corresponding electroactive material forms an electrode, and positioned between the two electrodes is an electrolyte that is ionically conductive and electrically insulative. In a multiple cell capacitor, several electrode and electrolyte layers can be formed between the two outer electrodes to provide the functionality of a plurality of capacitors within a single package.
It is desirable for solid electrolytes to be highly ionically conductive, stable at temperatures between 100 and 200.degree. Centigrade (C), minimally reactive, and low in cost. However, only a few known capacitor electrolytes, such as perfluorinated sulphonic membranes, e.g., Nafion by DuPont, and polybenzimidazoles (PBI), can be used at high temperatures, and these electrolytes are relatively expensive. Acid-doped solid electrolytes give good performance but are highly corrosive, requiring that current collectors used therewith be highly resistant to acid.
Thus, what is needed is an improved electrolyte for use in capacitors.